AFR 330.
Another way to explain, hope it helps.
1. A/C on ground, no power. You switch batteries (28vdc) and start APU.
2. APU ON. APU generator is powering whole a/c electricall system (115 vac) and part of the air compressed by APU compressor, necessary for APU own operation, is BLEEDED to supply a/c Pneumatic system (+/- 30 psig)
3. A/C pneumatic system feed air conditioning packs,,ENGINE STARTERS and several minors users. Depend of cockpit selections
4. MAIN ENGINE START: APU keep supplying electrical power to whole aircraft. All APU pneumatic supply is dedicated to one at a time engine start, (air conditioning is switched of during Engine Start). APU Bleed air is directed though air valves to a PNEUMATIC ENGINE STARTER where pneumatic pressure energy is transformed into mechanical torque that, through drive box, makes main engine rotor turn. When MAIN ENGINE ROTOR reaches certain speed, MAIN ENGINE Compressor is able to compress inlet air to a certain pressure at which combustion make take place, so fuel is inyected, ignition plugs are switched on and voilá -combustion. After a short time, MAIN ENGINE is capable of turning and acelerating on his own power, so Pneumatic Starter is mechanically disconected and no more pneumatic power is directed to this starter
Engine Ignition System - DutchOps.com powered
Of course there are differences, this is just a basic explanation. And sorry, I'm not an instructor
Keep asking!!!